1. Field of the Invention
This invention relates generally to optical systems and, in particular, to an optical system for a disposable rigid endoscope.
2. Discussion of the Prior Art
Endoscopes are optical instruments which permit the examination of body cavities without the need for extensive surgery. Conventional rigid endoscopes typically include a pipe for illuminating a region of the body cavity to be viewed and an optical lens system mounted in a tube for focusing and relaying the illuminated image from inside the body cavity to the physician. Essential optical components of a conventional lens system include a front combination of lenses constituting the objective lens, a system of relay lenses to carry the image through the scope, and an eye lens which produces a magnified virtual image for the viewer. Examples of such optical systems for endoscopes are described in U.S. Pat. Nos. 3,089,484 to Hett, 3,257,902 to Hopkins, 3,556,085 to Takahashi, 4,036,218 to Yamashita, 4,267,828 to Matsuo and 4,273,110 to Groux.
There are several drawbacks in the endoscopes of the prior art, specifically with regard to the complexity and expense of the optical systems incorporated therein. The optical components are typically made of glass that is ground and polished by expensive manufacturing techniques. As a result, the cost of these instruments is relatively high which thereby precludes disposing of the instrument after each surgery. It has become increasingly important to dispose of these instruments after each surgery to eliminate the risk of exposing the next patient to diseases such as AIDS or hepatitis. While most surgical instruments can be sterilized with high pressure steam, the delicate nature of optical systems in endoscopes makes such sterilization difficult. Also, since most of these instruments are inherently fragile, they are frequently broken, at great expense to the user.
In an effort to reduce the cost and complexity of the prior art systems, U.S. Pat. No. 4,784,118 to Fantone describes an endoscope in which the light pipe, and the objective, relay, and viewing lens assemblies are all made of polymeric materials, such as acrylics, polystyrenes, polycarbonates and styrene-acrylonitrile (SAN) copolymers, by conventional injection molding techniques.
Although the Fantone device offers some advantages over prior art optical systems, the device has several drawbacks. The polymeric relay lenses are composed of only one material which increases the chromatic aberration of the image and reduces the resolution. In addition, in order to obtain a bright image, the polymeric lenses have to be manufactured such that the length to diameter ratios of the lenses is relatively high. This is very difficult to achieve using currently known manufacturing techniques.
U.S. Pat. No. 4,964,710 to Leiner discloses a unique hybrid relay lens system that overcomes the problems with the Fantone endoscope and the other aforementioned prior art. This relay lens system incorporates glass plano cylinders disposed between molded polymeric curved surface lenses which have a thickness on the same order of magnitude as their diameter. The polymeric lenses are preferably fabricated from two different polymers to allow for the correction of chromatic aberration. In order to achieve a bright image, the plano glass cylinders with flat polished end faces are placed in between the polymeric lenses. In contrast to ground and polished lenses, the plano glass cylinders can be economically made in large quantities, while the smaller polymeric lenses can be economically and accurately made by known injection molding processes.
Despite the teachings of the prior art it remains desirable to provide an endoscope which is efficient and reliable to manufacture and which provides a clearer and brighter image while enhancing resolution. The present invention relates to a device which achieves these objectives by providing an improved optical system to be incorporated in a disposable rigid endoscope.